Device for amplifying the current of an abnormal electrical discharge and system for using an abnormal electrical discharge comprising one such device

ABSTRACT

A device (1) for amplifying the current of an abnormal electrical discharge, characterized in that it comprises an electrode which is positively polarized (2) and associated with a magnetic circuit (3) producing a magnetic field (4) which is uniformly divergent, whereby the intensity on the surface of the electrode is more than approximately 6.10 2  Tesla, the electrode being positioned in the region where the magnetic field is at its most intense.

[0001] The present invention relates to a device for amplifying thecurrent of an abnormal electrical discharge and a system for using anabnormal electrical discharge comprising one such device.

[0002] The invention relates to all devices for using the current of anabnormal electrical discharge, also known as a luminescent discharge,further known as a cold plasma, in particular for making deposits on,treating and modifying the surface of a metal or non-metal substrate.

[0003] Luminescent discharges are increasingly used as a material and/orenergy transfer medium in many surface modification processes; processesfor coating metal or non-metal parts may be cited, and among thesemagnetron cathode sputtering, plasma-enhanced chemical deposition,assisted thermochemical treatment processes such as post-discharge(deferred plasma) or ionic nitriding, ionic etching processes, plasmacleaning, etc. These techniques are used in most industrial fields,especially microelectronics, but also optics and mechanical engineering.

[0004] Luminescent discharges (cold plasmas) comprise a plurality oftypes of electrical discharge, each type having specificcharacteristics. The person skilled in the art refers to the electricaldischarges to which the invention relates as “abnormal discharges”; theyare characterized by the fact that in their domain of existence thecurrent varies as a monotonous function of the excitation voltage.

[0005] Devices for using the current of an abnormal electrical dischargeto treat a substrate (the term “treat” is to be understood asencompassing deposition on and modification of the surface of saidsubstrate) generally comprise an enclosure in which a vacuum can beestablished, since these discharges occur only at reduced pressure, asystem comprising an electrode (cathode) and an electrical power supplyto generate the discharge, and a substrate carrier designed to supportthe substrates to be treated. The current flowing through the substratedefines the number of ions that bombard it per unit time (ioniccurrent), and the voltage defines the energy of those ions. These twoparameters are important parameters for most processes.

[0006] One constraint imposed on such devices is the interdependence ofthe voltage and the ionic current.

[0007] Attempts to increase the ionic current are the outcome of seekingto improve the efficiency of the treatment. At low currents, this can beachieved only by increasing the voltage.

[0008] However, increasing the voltage can quickly become a problem, asions with too much energy can create unwanted modifications of thesurface of the substrate to be treated, coated or modified.

[0009] For example, a number of systems have been described forincreasing the flux of ions impinging on the substrates to be treatedwhen the magnetron cathode sputtering technique is used. The most widelyused is the unbalanced magnetron cathodes technique. The magneticconfiguration of the system forces the energetic electrons fleeing themagnetic trap of the cathode to converge toward the substrate, wherethey increase the ionization of the gas. The flux of ions obtaineddepends on the characteristics of the electrical discharge. Such systemshave been developed in particular by B. Window (see the papers by B.Window and N. Savvides, J. Vac. Sci. Technol. A 4 (3) May/June 1986,pages 453-456 “Unbalanced dc magnetrons as sources of high fluxes” andJ. Vac. Sci. Technol. A 4 (2) March/April 1986, pages 196-202 “Chargedparticle fluxes from planar magnetron sputtering sources”).

[0010] Devices for amplifying the current of an unbalanced magnetroncathodes electrical discharge cannot increase the ionic current on thesubstrate without in parallel increasing the ion acceleration voltage.

[0011] Systems have also been developed using auxiliary systems, forexample from magnetron cathodes to microwave excited plasma systems.

[0012] Systems have also been developed using a filament and a hollowcathode.

[0013] Such systems are not in widespread use today, mainly because theyare difficult to use and unreliable.

[0014] Furthermore, the prior art systems cannot, without greatdifficulty if at all, be produced with different geometries anddifferent sizes, in order to cater for a large number of industrialconfigurations.

[0015] One object of the invention is to provide a system for using thecurrent of an abnormal electrical discharge for treating a substratewhich increases the ionic current on the substrate without in parallelincreasing the ion acceleration voltage.

[0016] Another object of the invention is to provide a system that isreliable and easy to use for using the current of an abnormal electricaldischarge to treat a substrate.

[0017] A further object of the invention is to provide a system forusing the current of an abnormal electrical discharge that can easily beimplemented in a plurality of generic geometries, for example circularor rectangular, and with various dimensions.

[0018] Other objects and advantages of the invention will becomeapparent on reading the following description.

[0019] To meet the above objects, the present invention provides asystem for using an abnormal electrical discharge which includes aplasma source and is characterized in that it includes a new auxiliarydevice. The auxiliary device is based on using combined electric andmagnetic fields which, coupled with a plasma source, considerablyincreases the ionic current on the substrates independently of the ionacceleration voltage. A device of the above kind is referred tohereinafter as a device for amplifying the current of an abnormalelectrical discharge, an ionic amplifier or a plasma amplifier.

[0020] A device in accordance with the invention for amplifying thecurrent of an abnormal electrical discharge is characterized in that itcomprises an electrode which is positively polarized and associated witha magnetic circuit producing a magnetic field which is uniformlydivergent, whereby the intensity on the surface of said electrode ismore than approximately 6.10⁻² Tesla, said electrode being positioned inthe region where the magnetic field is most intense.

[0021] The magnetic field is measured at the surface and at the centerof the ionic amplifier.

[0022] The magnetic field preferably has an intensity from approximately8.10⁻² Tesla to approximately 25.10⁻² Tesla and even more preferablyfrom approximately 10.10⁻² Tesla to approximately 15.10⁻² Tesla.

[0023] Too low a magnetic field produces an unstable plasma whereas toohigh a magnetic field can be dangerous without being more effective.

[0024] The magnetic circuit can be made up of permanent magnets and/orelectromagnets, for example.

[0025] The dimensions of the device in accordance with the presentinvention for amplifying the current of an electrical discharge arerelated to the configuration of the utilization system in which it isinstalled. The positively polarized electrode can be a disk whosediameter is from approximately 50 mm to approximately 200 mm, forexample.

[0026] More generally, the positive electrode has a rectangular shapewhose width is from approximately 50 mm to approximately 200 mm andwhose height is from approximately 200 mm to approximately 1000 mm, forexample.

[0027] Although this is not limiting on the invention, the plasmaamplifier preferably includes a metal counter-electrode for stabilizingthe plasma.

[0028] The counter-electrode is a metal component that can be eitherconnected to ground or biased to a voltage from 0 to approximately −200V relative to ground.

[0029] The counter-electrode is positioned on the path of the electricaldischarge.

[0030] The person skilled in the art will readily be able to determinethe position of the counter-electrode.

[0031] The counter-electrode must be substantially parallel to thesurface of the amplifier, preferably at a distance from approximately 5mm to approximately 70 mm from the surface of the positively polarizedelectrode of said amplifier.

[0032] The counter-electrode can comprise a plate incorporating anorifice, for example a rectangular hole. The person skilled in the artwill understand that the dimensions of the orifice must be such that thedivergent magnetic field lines are not intercepted by the plate.

[0033] The counter-electrode can instead comprise a grid.

[0034] Finally, the counter-electrode can instead comprise a rod, forexample a rod with a diameter from 5 mm to 20 mm. In this case the rodis advantageously parallel to the central magnetic pole.

[0035] Depending on the embodiment of the invention, it may be necessaryto combine more than one type of counter-electrode in the sameamplifier.

[0036] The device in accordance with the invention for amplifying thecurrent of an electrical discharge can include a plurality ofcounter-electrodes of different types or the same type.

[0037] The counter-electrodes can be of the plate type, of the rod type,of the disk type, of the rectangular type or of the grid type, forexample.

[0038] The present invention also provides a system for using anabnormal electrical discharge, including a plasma source in an enclosurecontaining a gas at reduced pressure, characterized in that it includesin said enclosure a device in accordance with the invention as describedabove for amplifying the current of an electrical discharge.

[0039] The plasma sources are also referred to as “primary sources”.

[0040] The system according to the invention can include a plurality ofplasma sources. In this case, the system includes a plurality of devicesfor amplifying the current of an electrical discharge. Although this isnot limiting on the invention, each of the devices for amplifying thecurrent of an electrical discharge is preferably associated with onlyone plasma source.

[0041] The system according to the invention for using an electricaldischarge can be used to make a deposit on, treat or modify the surfaceof a substrate, for example. These uses are referred to by the genericterm “treatment” in the description.

[0042] In a preferred embodiment of the invention the plasma source orat least one of the plasma sources is a magnetron cathode sputteringcathode.

[0043] In this embodiment, if the positively polarized electrode of thedevice for amplifying the current of an electrical discharge isrectangular, the length of the positively polarized electrode must besubstantially the same as that of the magnetron cathode, in order toobtain a homogeneous plasma in the treatment area.

[0044] A system according to the invention for using an abnormalelectrical discharge generally includes a rotatable substrate carrierdesigned to support the parts to be treated. The substrate carrier canitself be negatively polarized to accelerate the positive ions emanatingfrom the source and thus bombard the substrate.

[0045] The disposition of the device for amplifying the current of anabnormal electrical discharge (plasma amplifier) must be such that theamplified plasma diffuses toward the substrate carrier. Also, it ispreferable to minimize the distance between the primary source and theplasma amplifier. Accordingly, in small deposition machines(diameter<400 mm), the dimensions are sufficiently small to position theplasma amplifier anywhere provided that the substrate carrier does notscreen in a penalizing manner the electrons emanating from the primarysource. In larger machines (diameter>400 mm), the plasma amplifier isadvantageously disposed at a distance of less than 400 mm from theprimary source and this distance is preferably reduced to the minimumpermitted by the mechanical construction. This configuration allowssatisfactory operation of the plasma amplifier because the concentrationof electrons emanating from the source is sufficient to ignite theamplified plasma. Also, this configuration minimizes interaction withthe substrate carrier. The electrons can then pass directly from thesource to the device for amplifying the current of an electricaldischarge.

[0046] As a general rule, the direction of the magnetic polarity of thedevice for amplifying the current of an electrical discharge has nosignificant effect on its effectiveness. However, when the source usesdevices based on magnetic fields to generate the plasma, the magneticpolarity of the amplifier must be adjusted relative to the primarysource, especially if the primary source is a magnetron cathodesputtering cathode. The central pole of the magnetron cathode and thedevice for amplifying the current of an electrical discharge mustpreferably have the same sign, so that no looping of field lines ispossible between the cathode and the device for amplifying the currentof an electrical discharge. It is increasingly important to comply withthis feature if the distance between the device for amplifying thecurrent of an electrical discharge and the magnetron cathode decreases(d<200 mm). If the distance is large (from 200 mm to 400 mm), thedirection of the magnetic polarity is no longer important, because theinteraction of the magnetic fields is then negligible.

[0047] The present invention further provides a method of treating parts(substrates) with a plasma, characterized in that it uses the systemdescribed above for using an abnormal electrical discharge.

[0048] In a method according to the invention, the electrical powersupply which supplies power to the device for amplifying the current ofan electrical discharge must have a minimum voltage for theamplification mechanism to be initiated. The positive voltage must besufficient (in practice greater than approximately 20 V) for theelectrons to be able to ionize the gas. When amplification commences,the positive voltage of the amplifier changes very little when theimposed current changes over a large range. Depending on what isrequired, the power supply can provide a direct current or a currentchopped at a low, medium or high frequency. If the source is a magnetroncathode sputtering cathode, the maximum current imposed on the plasmaamplifier must not exceed 1.5 times that of the source, regardless ofthe mode of polarization of the latter. Beyond 1.5 times the cathodecurrent, the amplified plasma becomes very unstable and generateselectrical arcs on the various components of the deposition system, andthe voltage of the amplifier begins to increase greatly (greater thanapproximately 60 V) and fluctuates intensely. Moreover, if the voltageof the plasma amplifier becomes too high (>60 V), the walls of thedeposition machine begin to be pulverized, which degrades the quality ofthe process.

[0049] A second feature of the method according to the invention isconsequently that the supply voltage of the device for amplifying thecurrent of an electrical discharge must be from approximately 20 voltsto approximately 60 volts.

[0050] The plasma amplifier according to the invention has manyadvantages. In particular, the plasma amplifier increases the ioniccurrent on the substrate carrier by a factor from 1 to 10 compared tothe current normally obtained with conventional magnetron cathodesputtering. The factor depends on the current imposed on the amplifier(from 0 to 1.5 times the cathode current).

[0051] The invention also has other advantages that are particularlybeneficial for the user: the ionic current from the plasma amplifier canbe adjusted independently of that from the primary source. Also, thebias voltage and the current of the substrate carrier can be adjustedindependently, so that it is possible to control independently thenumber and the energy of the ions bombarding the substrate.

[0052] The invention is described in more detail next, but withoutlimiting the invention, with reference to the accompanying drawings, inwhich:

[0053]FIG. 1 is a diagrammatic view of a device according to theinvention for amplifying the current of an abnormal electricaldischarge,

[0054]FIG. 2 is a diagrammatic view of an embodiment of a deviceaccording to the invention for amplifying the current of an abnormalelectrical discharge including a counter-electrode in the form of aplate, and

[0055]FIG. 3 is a diagrammatic view of an embodiment of a deviceaccording to the invention for amplifying the current of an abnormalelectrical discharge including a counter-electrode in the form of a rod.

[0056] Referring to FIG. 1, the device 1 in accordance with theinvention for amplifying the current of an abnormal electrical dischargeincludes a rectangular electrode 2 which is positively polarized,associated with a magnetic circuit 3 producing a magnetic field 4 whichis uniformly divergent, whereby the intensity on the surface of saidelectrode is more than approximately 6.10⁻² Tesla. The electrode 2 ispositioned in the region where the magnetic field 4 is most intense.

[0057] Referring to FIG. 2, the device 1 according to the invention foramplifying the current of an electrical discharge further includes ametal counter-electrode 5 in the shape of a plate positioned on the pathof the electrical discharge and substantially parallel to the surface ofthe amplifier device 1.

[0058] Referring to FIG. 3, the device 1 according to the invention foramplifying the current of an electrical discharge further includes ametal counter-electrode 5′ in the form of a rod positioned on the pathof the electrical discharge and substantially parallel to the surface ofthe amplifier device 1.

[0059] The following example serves to illustrate the invention withoutlimiting it.

EXAMPLE

[0060] In a 350 mm diameter vacuum deposition machine, a 150 mm diametermagnetron cathode is installed and supplied with a power of 1800 W at avoltage of 550 V (3.3 A). The plasma amplifier receives a supply currentof 4 A. The rotating substrate carrier is biased to a voltage of −100 V.The bias current Ib is measured.

[0061] The magnetic field at the surface of the amplifier is 11. 10⁻²Tesla.

[0062] The electrode is a 150 mm diameter disk.

[0063] The ion acceleration voltage V is measured with and without theplasma amplifier according to the invention (configuration 0 andconfiguration 1, respectively).

[0064] The efficacy of the system is characterized by the factor k, theratio of the current Ib_(A) collected on the parts when the plasmaamplifier is operating and the current Ib received when there is noplasma amplifier.

[0065] The results are indicated in Table I below. TABLE I Plasma V involts amplifier on substrate Configuration present k = Ib_(A)/Ib carrier0 (comparative) No 1 −100 1 Yes 6.2 −100

[0066] Note that the plasma amplifier according to the inventionincreases the ionic current on the substrate without in parallelincreasing the ion acceleration voltage V.

[0067] The person skilled in the art will understand that although theinvention has been described and shown by means of particularembodiments, numerous variants can be envisaged without departing fromthe scope of the invention as defined in the accompanying claims.

1. A device (1) for amplifying the current of an abnormal electricaldischarge, characterized in that it comprises an electrode (2) which ispositively polarized and associated with a magnetic circuit (3)producing a magnetic field (4) which is uniformly divergent, whereby theintensity on the surface of said electrode is more than approximately6.10⁻² Tesla, said electrode being positioned in the region where themagnetic field is most intense.
 2. A device (1) according to claim 1 foramplifying the current of an abnormal electrical discharge,characterized in that the magnetic field (4) has an intensity fromapproximately 8.10⁻² Tesla to approximately
 25. 10⁻² Tesla.
 3. A device(1) according to claim 2 for amplifying the current of an abnormalelectrical discharge, characterized in that the magnetic field (4) hasan intensity from approximately 10.10⁻² Tesla to approximately 15.10⁻²Tesla.
 4. A device (1) according to any of claims 1 to 3 for amplifyingthe current of an abnormal electrical discharge, characterized in thatthe positively polarized electrode (2) is a disk.
 5. A device (1)according to claim 4 for amplifying the current of an abnormalelectrical discharge, characterized in that the disk has a diameter fromapproximately 50 mm to approximately 200 mm.
 6. A device (1) accordingto any of claims 1 to 3 for amplifying the current of an abnormalelectrical discharge, characterized in that the positively polarizedelectrode (2) is rectangular.
 7. A device (1) according to claim 6 foramplifying the current of an abnormal electrical discharge,characterized in that the rectangle is from approximately 50 mm toapproximately 200 mm wide and from approximately 200 mm to approximately1000 mm high.
 8. A device (1) according to any of claims 1 to 7 foramplifying the current of an abnormal electrical discharge,characterized in that it includes a metal counter-electrode (5, 5′)positioned on the path of the electrical discharge and substantiallyparallel to the surface of said amplifier device.
 9. A device (1)according to claim 8 for amplifying the current of an abnormalelectrical discharge, characterized in that the counter-electrode (5,5′) is positioned at a distance from approximately 5 mm to approximately70 mm from the surface of the positively polarized electrode of saidamplifier device.
 10. A device (1) according to either claim 8 or claim9 for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is a plate.
 11. A device (1)according to claim 10 for amplifying the current of an abnormalelectrical discharge, characterized in that the plate has an orificethrough it.
 12. A device (1) according to claim 11 for amplifying thecurrent of an abnormal electrical discharge, characterized in that theorifice is rectangular.
 13. A device (1) according to either claim 8 orclaim 9 for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is a rod.
 14. A device (1)according to claim 13 for amplifying the current of an abnormalelectrical discharge, characterized in that the rod has a diameter fromapproximately 5 mm to approximately 20 mm.
 15. A device (1) according toeither claim 8 or claim 9 for amplifying the current of an abnormalelectrical discharge, characterized in that the counter-electrode is adisk.
 16. A device (1) according to either claim 8 or claim 9 foramplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode is rectangular.
 17. A device(1) according to either claim 8 or claim 9 for amplifying the current ofan abnormal electrical discharge, characterized in that thecounter-electrode is a grid.
 18. A device (1) according to claim 8 foramplifying the current of an abnormal electrical discharge,characterized in that it includes a plurality of counter-electrodes. 19.A device (1) according to claim 18 for amplifying the current of anabnormal electrical discharge, characterized in that thecounter-electrodes are of different types.
 20. A device (1) according toclaim 19 for amplifying the current of an abnormal electrical discharge,characterized in that the counter-electrode types are chosen from theplate type, the rod type, the disk type and the grid type.
 21. A systemfor using an abnormal electrical discharge, including a plasma source inan enclosure containing a gas at a reduced pressure, characterized inthat it includes in said enclosure a device according to any of claims 1to 20 for amplifying the current of an abnormal electrical discharge.22. A system according to claim 21 for using an abnormal electricaldischarge, characterized in that it includes a device for amplifying thecurrent of an abnormal electrical discharge for the plasma source or foreach plasma source, the or each device for amplifying the current of anelectrical discharge being associated with only one plasma source.
 23. Asystem according to either claim 21 or claim 22 for using an abnormalelectrical discharge, characterized in that at least one of the plasmasources is a magnetron cathode sputtering cathode.
 24. A systemaccording to any of claims 21 to 23 for using an abnormal electricaldischarge, characterized in that said use is chosen from deposition onand treatment and modification of the surface of a substrate.
 25. Asystem according to claim 24 for using an abnormal electrical discharge,characterized in that it is associated with a rotatable substratecarrier.
 26. A system according to claim 25 for using an abnormalelectrical discharge, characterized in that the substrate carrier isnegatively polarized.
 27. A method of treating substrates with a plasma,characterized in that a system according to any of claims 21 to 26 forusing an abnormal electrical discharge is used and in that the positivesupply voltage of the device for amplifying the current of an abnormalelectrical discharge is from approximately 20 volts to approximately 60volts.